1. Field of the Invention
The present invention relates generally to ultrasonic nondestructive testing and, more particularly, to a phased array ultrasonic testing system for inspecting turbine blade attachments and disc bores. The invention also relates to methods of examining turbine blades and disc bores, and modeling turbine components of unknown geometry using a phased array ultrasonic testing system.
2. Background Information
Gas and steam turbines for electrical power generation must be routinely inspected in order to detect discontinuities, such as stress corrosion cracking (SCC). SCC can result from the combination of high operational forces and prolonged exposure to a corrosive environment. Two portions of the turbine which are areas of relative stress concentration and, therefore, are especially susceptible to SCC, are the blade attachments, where the base or root of the turbine blades attach to the turbine disc, and the bore of the turbine disc. Defects in these and other areas must be identified before they progress to a point where they could result in component failure.
Non-destructive evaluation (NDE) methods, such as ultrasonic testing (UT), are typically employed to inspect turbine blade attachments and disc bores. Ultrasonic testing is generally old and well known in the art. In general, high frequency sound waves are applied to the structure being tested using one or more transducers. The transducers typically comprise piezocrystal elements that are excited by an electrical voltage in order to induce the ultrasonic waves in the structure. When the sound waves interact with something (e.g., a void; a crack or other defect) having a significant difference in impedance from that of the propagation medium, a portion of the sound is either reflected or diffracted back to the source from which it originated. Detection and quantification of the returned sound pattern is used to determine the characteristics of the reflecting medium. The concepts of ultrasonic testing and, in particular, phased array ultrasonic technology, are explained in further detail in the book Introduction to Phased Array Ultrasonic Technology Applications, by Dr. Michael D. C. Moles et al., R&D Tech Inc., 2004.
Phased array ultrasonic technology generally provides for the computer-controlled excitation (e.g., amplitude and delay) of individual elements in a multi-element probe (as opposed to single-element probes of conventional UT). The excitation of piezocomposite elements can generate a focused ultrasonic beam with the potential to modify beam parameters such as angle, focal distance, and focal point, through software. Thus, a computer-controlled beam scanning pattern can be implemented in order to “steer” (e.g., direct) the beam to the area of interest and to search for cracks or other discontinuities.
FIG. 1 is a cross-sectional simplified view of a representative example of a turbine rotor assembly 1. The rotor assembly 1 generally includes a shaft 3 having a plurality of discs 5 mounted coaxially thereon. The shaft 3 extends through the bore formed at the center of each disc 5. A plurality of blades 7 are mounted to the periphery of each disc 5. In the example of FIG. 1 the blades 7 are mounted by insertion of a root portion 9 of the blade 7 formed along the circumference of the disc 5. This area is generally referred to as the blade attachment 9. As previously discussed, both the bores of the discs 5 and the blade attachments 9 must be routinely inspected.
Turbine components and, in particular, blade attachment and disc designs and configurations can differ significantly among the various manufacturers in the power generation field. By way of example, most, if not all, turbine blades are attached to the discs using one of two known blade attachment configurations, a side-entry (e.g., generally perpendicular to the shaft axis) configuration commonly referred to as a straddle-mount configuration, or an axial configuration wherein the blades attach to the disc in a direction which is generally parallel to the axis of the shaft. Axial blade attachments and associated discs have much more complicated geometries than their straddle-mount counterparts. Specifically, unlike axial configurations in which the blade attachments and the discs in general have a number of compound curves including curved, contoured, and otherwise irregular geometries, straddle-mount attachments have a relatively simple geometry substantially devoid of compound curvature, for example, and instead consist of a series of substantially straight mounting (grooves. The associated straddle-mount discs are also relatively simple in shape. For example, the sides of the disc are generally straight or flat between the blade attachment area and the disc bore. Straddle-mount blade attachments and discs therefore, are available from the General Electric Company which has a place of business in Niskayuna, N.Y. The axial entry design is available from Siemens Westinghouse Power Corporation which has a place of business in Orlando, Fla.
Inspection using ultrasonic testing techniques gets more and more difficult as the complexity of the geometry of the object to be tested increases. For instance, compound curves make ultrasonic testing very difficult because one portion of the compound curve may, for example, be convex and therefore function to diverge the ultrasonic wave being projected by the transducer while another portion may, for example, be concave and therefore tend to converge the beam. Both the axial mount blade attachments and the sides of the associated discs, for example, have at least one compound curve. As a result of the complexity of the design, commercially available ultrasonic inspection has been limited. NDE has, therefore, been largely limited to surface sensitive techniques such as magnetic particle, dye penetrant, or eddy current. Accordingly, it is appreciated that examining Siemens Westinghouse discs and blade attachments is more difficult than examining discs and blade attachments of other manufacturers. Some of the additional difficulties associated with inspection of Siemens Westinghouse disc bores are outlined in the paper entitled “SWPC Disc Bore Inspection Method: Challenges Inspecting Siemens Westinghouse Disc Bores,” Siemens Westinghouse.
Although there have been many attempts to apply various ultrasonic testing techniques to the inspection of turbine components, there remains a very real and substantial need for an improved ultrasonic testing system capable of accommodating the complex geometry of for example, axial entry turbine blade attachments and turbine discs.
For example, with respect to inspection of blade attachments, U.S. Pat. No. 6,082,198, discloses a method of using phased array ultrasonic sensors mounted on one of the turbine disc hubs in order to inspect the opposite face. The method is intended to simultaneously reconstruct and test straddle-mount turbine hubs with the turbine blades in place. However, the method is limited in application to the relatively simple geometry of the straddle-mount design, as previously discussed. The disclosed probe mounting location and scanning methods would not sufficiently accommodate the complex geometry of, for example, the axial blade attachment design to provide accurate and reliable inspection results.
With respect to ultrasonic inspection of turbine discs, U.S. Pat. No. 6,736,011, for example, discloses a linear ultrasonic array probe for detecting and inspecting for SCC in the area of the disc bore and keyway of shrunk-on steam turbine discs. However, the method is generally limited to linear or one-dimensional inspection probes and methods, and to the requirement that the probes be placed on the disc at a location across from the keyway area.
There is, therefore, room for improvement in systems and methods for ultrasonic examination of turbine blade attachments and disc bores, and in methods of modeling and examining turbine components of unknown geometry.